Are Particle Counters and Government Machines the Same?

My particle counter is a beast–I’ve loved it. But I’ve always wondered how the counts of laser particle counters like mine:

…compare to the measurements of the huge stationary air quality monitoring stations that governments use, like this one in New Zealand:

If you look at the US Embassy’s Twitter feed, you’ll notice that its raw numbers are “concentration.” These are micrograms per cubic meter (µg/m³). The way government (BAM) machines work is that they use a source of carbon 14 that emits beta particles. Then they measure how many of those beta particles make it through a dirty filter and to a detector. They then use those numbers to estimate the weight of those particles (micrograms).

Weight vs. Number

In contrast, laser particle counters like mine use a laser and a photo diode sensor to estimate the number of particles in the air. I don’t see why the weight should be any more important than the number. They’re both telling you how much particulate pollution is in the air.

As an analogy, imagine we want to understand the crowd at a basketball game. We could count the number of people, or we could weigh those people. Of course, the more people, the heavier the total weight. And of course, the two numbers won’t correlate perfectly. We might have more heavy people on some days and more children on other days. But the weight and the total number should correlate highly.

The other major difference is that laser particle counters give the number of particles at that particle size and above. Government machines give the number of particles at that size and below.

The Dylos Comparison Test

To see how the two numbers compare, I put my particle counter outside my second-story window 70 times (that’s nerd dedication!). Then I compared my numbers to the US Embassy’s Twitter feed at the same time. Here’s what they look like:

They correlate at r = .89, meaning the two numbers are very strongly related (remember the highest possible correlation is 1). That high correlation is especially impressive given that my house is near Gulou, and the US Embassy is out in Liangmaqiao–about 7 kilometers away.

The difference between the readings was particularly noticeable on days where a strong wind moved through Beijing. I noticed several times that my particle counts would drop before the embassy’s counts as the wind moved in from the west (where my house was) to the east (where the embassy is). (Remember, Beijing’s air gets a lot cleaner when we get winds from the west.)

Converting The Dylos to Micrograms

If we want to get a rough conversion between the numbers, we can remove a few of the outliers and compute a regression line. On that line, a government concentration of 100 micrograms (four times the WHO limit!) is approximately 25,000 on the Dylos particle counter:

And the 24-hour WHO standard of 25 µg/m³ is about 3,000-4,000 on the Dylos.

During my home tests (before turning my filter on), the air inside my home was very often above 3,000. That’s despite the fact that it was still much cleaner than outside.

Conclusion: The Dylos particle counter is giving measurements that are highly related to the much larger air monitoring stations. The scale is different, but the two can be roughly converted.

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Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

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Ryan Rose

For a DIY filter, which HEPA do you recommend? (Shipping is just too discouraging.) We checked Amazon but they’re out of stock, as your webpage also says (12-1-19).
Have you or others factored in or out the effects of humidity? I live in the Southwest US where humidity is almost always low, which it seems to me should be a positive in terms of mold, etc., growing on and eventually blowing out of the filter medium.

Hey Ryan! If you’re after our HEPA filters, you can pick them up from our shop here. They should be in stock! Which HEPA to go for really depends on how big your fan is. However, since you’re based in the US you could get yourself big blast furnace filters. They’re not quite as effective as HEPAs at capturing pollution in one pass, but still do a great job and over a short period of time work just as well as HEPAs. You should be able to pick them up very cheaply of Amazon, and no need to spend on… Read more »

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